The invention relates generally to wireless navigation and more particularly to radio navigation using, for example, the Global Positioning System (GPS).
Recent technological developments combined with the economic advantages of mass production have resulted in the proliferation of small, lightweight navigational receivers capable of accurately determining a user""s position, distance, and bearing using wireless broadcast signals. One particularly important example of such a wireless receiver operates using a navigational system known as the Global Positioning System (GPS).
Developed initially for military applications, the GPS includes a space segment of orbiting satellites, each broadcasting a wireless navigation signal. GPS includes a sufficient number of satellites, configured as a constellation, to provide global coverage. GPS presently enables numerous applications ranging from automobile navigation to retrieval of stolen cars to precisely guiding munitions to their targets.
Conventional GPS navigational receivers can provide precise location information under certain operational conditions by receiving and processing the navigational signals from a sufficient number of the GPS satellites. The GPS system is a range-based navigational system whereby a receiver""s geographical position is uniquely determined by independent range measurements to a sufficient number of GPS satellites. The GPS receiver determines its location from the measured range to each satellite, the satellite""s identity, and its location. The radio-frequency (RF) navigational signal broadcast by each of the GPS satellites is modulated with a unique code (a series of 1""s and 0""s that repeats over a long period in relation to the data rate). The navigational signal of each satellite also periodically broadcasts information identifying its approximate position (e.g., the satellite""s ephemeris data).
The range represents the straight-line distance (e.g., in meters) between the receiver and the satellite. The range is generally calculated by measuring a propagation delay (i.e., the time it takes for the signal to propagate from the satellite to the receiver) associated with the received navigational signal. The receiver then typically determines the range as the product of the measured propagation delay and an assumed propagation velocity. For example, the propagation velocity is generally known for the particular geometry of the radio-frequency (RF) path. The propagation velocity would be the speed of light in a vacuum if the path were, in fact, through a vacuum; however, the propagation velocity is typically a lesser (but approximately determinable) value due to the effect of the earth""s atmosphere.
Generally, GPS receivers are designed to operate with more than the minimum number of satellites xe2x80x9cin viewxe2x80x9d (i.e., in direct line-of-sight) of the receiver that would be necessary to support a full navigation solution. For example, range measurements at the receiver to three GPS satellites would indicate the receiver""s location at either of two points on the earth""s surface. Range measurements to additional satellites would increase the accuracy of the determined position. Conversely, blockage of the direct path to one or more of the satellites (e.g., obstruction of the direct, or line-of-sight, path by mountains or buildings) may compromise the receiver""s ability to determine and maintain awareness of its position, particularly where the blockage results in position determination using a lesser number of satellites than minimally required.
GPS receivers must also deal with reflections of the navigation signals originating from one or more of the satellites. For example, obstacles such as buildings can block the direct path between the receiver and satellite, so the signal reaching the receiver is a result of one or more reflections. These RF reflections are commonly referred to as xe2x80x9cmultipathxe2x80x9d signals. Where the GPS receiver is able to receive a direct-path signal, the multipath signals represent unwanted noise, or interference. Some GPS receivers are able to discard the multipath under certain conditions. But when the GPS receiver receives one or more multipath signals yet is unable to maintain direct-path communications, the receiver may inadvertently process the multipath as though it were the direct path signal, resulting in erroneous range measurements. Of course, even if the erroneous ranges resulting from the multipath are identified and discarded, the absence of a usable navigational signal precludes position determination.
Unfortunately, many areas where GPS receivers operate (e.g., urban environments) inevitably result in both blockage of the direct paths and the occurrence of multipath signals. Some prior-art GPS receivers have been designed to xe2x80x9ccoastxe2x80x9d (i.e., refrain from further position computation) when fewer than the full number of satellites are in view, while other prior-art receivers utilize navigational information from other sources in the absence of a sufficient number of direct-path satellite signals. However, GPS receivers are generally unable to maintain their accuracy while coasting, and solutions relying on other navigational systems can be costly and more complex, thus defeating the objective of providing a small, light-weight, and inexpensive system.
In general, the present invention relates to wireless navigational systems, such as GPS, that utilize multipath wireless signals normally discarded from the navigation solution to enhance a navigational system""s performance under a range of operational scenarios. The navigational system may provide solutions where direct paths between a transmitter, such as a satellite, and a receiver become obscured, or blocked. The present invention may also be used in conjunction with other navigational techniques to provide a more robust navigational capability.
Accordingly, in a first aspect, the invention relates to a process for navigating using a multipath wireless signal whereby a receiver receives navigational information indicative of a geographical location. The receiver also receives a multipath navigation signal, which the receiver relates to the reliably computed geographical location. Having determined the relationship, the receiver is able to perform navigation using the multipath signal and the predetermined relationship between the multipath signal and the geographic location.
In one embodiment the receiver receives navigational information by way of a direct-path wireless signal. The receiver computes a geographic location and relates a multipath navigation signal to the geographical location by correlating the multipath signal with the direct-path signal. In some embodiments an offset coefficient relating the multipath signal to the direct-path signal is determined, whereby processing the offset coefficient with the multipath signal facilitates the determination of a current location.
In another embodiment the step of determining a path delay comprises the steps of time-shifting a replica of a received signal, correlating the received signal with the time-shifted replica thereof, and determining a time-shift value resulting in a relative maximum correlation of the received signal with the time-shifted stored representation of the received signal.
In another embodiment wherein at least one of the multipath signal and the direct-path signal is of insufficient strength to permit navigation based thereon, the navigation step includes combining the multipath signal and the direct-path signal to provide a navigation solution.
In yet another embodiment the current location is determined by a wireless multipath signal generated by a GPS transmitter. In other embodiments the current location is provided by another navigational system, such as an inertial navigational system. In yet other embodiments, the current location is provided by a known starting location, such as map coordinates.
In another aspect, the invention relates to an apparatus for navigating using a multipath wireless signal. The apparatus includes circuitry for providing a geographical location, a receiver for receiving a wireless multipath navigation signal, circuitry for relating the multipath signal to the geographical location, and circuitry for navigating using the multipath navigation signal based on the relationship.
In some embodiments the receiver includes circuitry configured to receive a spread-spectrum signal, such as a code-division multiple-access (CDMA) signal. In one embodiment the receiver is configured to receive the wireless multipath navigation signal from a GPS transmitter.
In one embodiment the relating circuitry comprises a correlator interactive with the geographic location circuitry and the receiver. The correlator provides an output indicative of a relationship between the geographic location and the received wireless multipath navigation signal. In some embodiments, the correlator may be part of a tracking loop filter that provides an indication of the delay.
In another embodiment the navigating circuitry includes a processor in communication with the relating circuitry. The processor processes an output signal received from the relating circuitry; the output signal relates the multipath navigation signal to the current geographic location. Based on the relationship between the multipath navigation signal and the current geographic location, the processor determines a current location using the wireless multipath navigation signal.
In another embodiment the relating circuitry includes circuitry for determining a first path delay corresponding to a direct-path range. The relating circuitry also includes circuitry for similarly determining a second path delay corresponding to a multipath range. Having determined the direct path delay and multipath delay(s), the circuitry determines an offset coefficient relating the first path delay to the second path delay.
In another embodiment the circuitry for determining the first and second path delays time-shifts a stored representation of a received signal. The circuitry correlates the received signal with the time-shifted stored representation thereof, and determines a time-shift value corresponding to a relative maximum correlation. In some embodiments the delay device includes a shift register. In other embodiments the delay device includes a tapped-delay line.
In another embodiment, where the amplitude of at least one of the received multipath signal and the received direct-path signal is insufficient to facilitate navigation, the navigational circuitry includes circuitry that combines the received signals to provide a navigation solution.
In yet another embodiment the geographical-location circuitry is configured to determine the geographical location from a known starting location and an estimate of a current location based on the starting location.